Hitherto, there has been known a seat back for a vehicle seat, in which a seat back frame made of recyclable light alloy such as magnesium alloy, aluminum alloy or the like is employed (Japanese Patent Application Laid-Open No. 2001-114004).
The conventional seat back frame that is made of light alloy includes a frame body that is formed into a substantially inverted U-shape in outline. Recliner means for allowing the seat back to be pivoted with respect to a seat cushion of the vehicle seat is attached to a lower part of the seat back frame. The frame body includes left and right side sections that are spaced apart from each other, and an upper section interconnecting the left and right sections. The frame body further has a lower section that is formed into a flat plate-shape having a width, extending between lower regions of the left and right side sections and interconnecting the lower regions of the left and right side sections.
However, the lower section of the seat back frame that is merely formed into a flat plate-shape having a width is undesirable since when the lower section is subjected to any external force that may be applied to the seat back frame by, for example, crashing, and should the lower section be broken, a broken portion of the lower section may be excessively projected.
Similarly, the lower section of the seat back frame is undesirable in terms of stiffness or resistance to the external force that may be applied to the seat back frame by, for example, the crashing. In order to provide sufficient stiffness for the lower section, the lower section may be heavy-sectioned. However, this is contrary to a demand for weight-lightening of the seat back frame.